The US, UK and France have launched more than 100 missiles against what they say were Syrian chemical weapons facilities in response to a chemical weapons attack in a Damascus suburb a week ago.

The Pentagon said the strikes, which began at 4am Syrian time (0200 GMT), involved planes and ship-launched missiles and identified three targets: a scientific research centre in Damascus, a chemical weapons storage facility west of Homs, and another storage site and command post nearby.

Announcing the launching of the action in a seven-minute speech, President Donald Trump said the US was prepared to sustain economic, diplomatic and military pressure on Syria’s president, Bashar al-Assad, until he ends what he called the criminal pattern of killing his own people with chemical weapons.

“The evil and the despicable attack left mothers and fathers, infants and children, thrashing in pain and gasping for air. These are not the actions of a man; they are crimes of a monster instead,” Trump said.

The Russian president, Vladimir Putin, condemned the airstrikes, saying they would add to the humanitarian catastrophe in Syria, adding that Moscow was calling for an emergency session of the UN security council to debate the military action.

He called the strikes an “act of aggression” that had a “destructive influence on the entire system of international relations”.

China and Iran joined in the condemnations of the strikes. Syria’s president Assad said would increase Syria’s resolve to “fight and crush terrorism in every inch” of the country.

Highlighting the limited nature of the raids – and the desire to avoid a dangerous escalation – the US defence secretary, James Mattis, said: “Right now this is a one-time shot”. The French defence minister, Florence Parly, said Moscow had been warned by France and its allies about the strikes beforehand.

The UN secretary general, António Guterres, said he was closely watching reports of the attack and told the countries involved that they were obliged to act within the guidelines of the charter of the United Nations and “international law in general”.

The Russian military claimed Syrian air defences had shot down some of the incoming missiles, including 12 aimed at an airbase, claims that could not be verified. Russia said it had not engaged its own air defences at its air and naval bases in Syria.

As large detonations were visible in several parts of the war-torn country, Syrians crowded on to the streets in noisy demonstrations of defiance and their Russian ally denounced the attack.

After Trump finished his seven-minute address, Theresa May and Emmanuel Macron made separate announcements of British and French participation, stressing that the strikes were limited to Syrian regime chemical facilities, and had no wider goals.

May said there was no alternative to the action the three countries were taking.

Explosions were reported in Damascus moments after Trump’s address. Later, a Syrian official said all sites had been evacuated days ago after a warning from Russia.

The Russian ambassador to Washington, Anatoly Antonov, issued a statement threatening “consequences”. “A pre-designed scenario is being implemented,” the statement said. “Again, we are being threatened. We warned that such actions will not be left without consequences. All responsibility for them rests with Washington, London and Paris.”

None of the strikes hit zones where Russian air defence systems protect the Russian bases of Tartus and Hmeimim, Russian news agencies cited the Ministry of Defence as saying.

The Pentagon said in the immediate aftermath of the strikes that while there had been some Syrian air defence fire, it was not clear whether Russian air defences in Syria had responded.

Syrian state TV appears to show aftermath of airstrikes – video

The attack came on the eve of a planned visit by inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) to the site of last week’s chemical weapons attacks, the Damascus suburb of Douma. The US, UK and France had announced they had reached their own conclusion that the Syrian regime was responsible, an accusation denied by Damascus and Russia, which claimed on Friday the attack had been staged by British intelligence.

The White House produced a summary of the evidence it said pointed to the regime’s responsibility for the Douma attack. It said that regime helicopters were seen by witnesses hovering over the area of the attack on 7 April dropping barrel bombs. Remnants of the barrel bombs looked like “chlorine barrel bombs from past attacks”. It said that the victims showed symptoms of both chlorine and sarin poisoning.

The White House assessment also claimed there was “reliable information indicating coordination between Syrian military officials before the attack”.

‘We have sent a clear message’

Mattis, who had said on Thursday the US was still looking at the evidence, said on Friday night he was “absolutely confident” that the regime was responsible for use of poison gas in Douma. He said there was clear evidence of the use of chlorine, but “we are not certain about sarin right now”.

Mattis said the “Assad regime clearly did not get the message last year” when the US launched a Tomahawk missile strike at a desert airbase following a poison gas attack in April 2017. On that occasion, 57 missiles were fired. Mattis said slightly more than double that total were used in the strikes overnight.

“This time … we have struck harder,” the defence secretary said. “Together we have sent a clear message to Assad and his murderous lieutenants that they should not perpetrate another chemical weapons attack for which they will be held accountable.”

Trump said the Douma attack was “a significant escalation in a pattern of chemical weapons use by that very terrible regime”. He noted that establishing deterrence against use of such weapons represented “a vital national security interest of the US”.

Macron confirmed that France was involved in the airstrikes, saying the French role would be limited to Syria’s chemical weapons facilities.

“We cannot tolerate the recurring use of chemical weapons, which is an immediate danger for the Syrian people and our collective security,” a statement from the Élyséepresidential office said.

May stressed that the aims of the intervention were limited to stopping chemical weapons use, for humanitarian reasons, and to uphold the international norm outlawing their use.

“We have sought to use every possible diplomatic channel to achieve this. But our efforts have been repeatedly thwarted,” she said, pointing to a Russian veto at the UN security council on a proposal to set up a new investigative body for chemical weapons incidents in Syria.

“So there is no practicable alternative to the use of force to degrade and deter the use of chemical weapons by the Syrian regime. This is not about intervening in a civil war. It is not about regime change.”

The decision to launch a military strike in response to last Saturday’s chemical weapons attack in a rebel-held district of Damascus was fraught with risks. There are Russian and Iranian forces in bases across Syrian and substantial Russian air defences in the west of the country. Russian officials had threatened to use those defences.